The present invention relates to a ferrule for optical fiber connectors used for connecting optical fiber cables, and a method for injection molding such a ferrule. In particular, the present invention relates to a plastic ferrule and a method for making the same which allow a ferrule having highly precise external dimensions and coaxiality to be mass produced easily and economically by means of an injection molding process using an insert pipe.
Ferrules of this kind are disclosed in Japanese patent laid open publications Nos. 2000-111758 and 2001-96570. According to these ferrules, an insert pipe consisting of a metallic pipe is fitted onto the front end of the ferrule provided with a core wire receiving bore for receiving an extremely fine optical fiber core wire, and this pipe extends to at least a flange portion provided in an axially intermediate part of the ferrule.
Such a ferrule can be made by placing an insert pipe provided with communication holes for passing plastic material in a part thereof corresponding to a flange portion in a metallic die assembly, placing core pins in an axial center for defining bores for receiving a core wire and a sheath at an end of an optical fiber, and injecting plastic material from a gate provided near the communication holes so as to pass the plastic material into the communication holes. The entire ferrule including the flange portion can be thus injection molded as an integral assembly.
Because such a ferrule is injection molded by using an insert pipe, a required level of circularity can be ensured for the outer circumference of the front end of the ferrule, and the precision in external dimensions and coaxiality can be improved. Also, a favorable registration can be achieved when connecting the ferrule to an opposing ferrule by using a sleeve commonly fitted over the two ferrules, and the positional precision for the optical core wire which is fitted into the core wire receiving bore can be improved. These factors contribute to the reduction in the transmission loss.
However, such a ferrule is not without some problems that are desired to be resolved. First of all, the core wire receiving bore may not be provided with a uniform diameter, and tends to be tapered. This not only causes difficulty in inserting an optical fiber core wire but also may cause cracks and deformations in the core wire. Cracks and deformations in the core wire increase transmission loss and reflective attenuation, and prevent the light signal from traveling along a linear straight path. These factors severely impair the performance of the optical fiber connector.
When the core pin is removed from the core wire receiving bore immediately after the ferrule is molded and knocked out from the metallic die assembly, the plastic material undergoes a thermal contraction which tends to be more pronounced toward the front end at which the insert pipe opens out so that the core wire receiving bore tends to be tapered toward the front end. The inventors have discovered that this tendency gets pronounced particularly when the mass of the plastic material in the front end is large or the wall thickness is great.
When molding a ferrule, the plastic material is typically injected from a gate provided near the intermediate flange portion, and then flows toward the front end. The front part of the flow of the plastic material includes a higher component of slag or other non-uniform part of the plastic material, and eventually reaches the front end portion of the ferrule in which the core wire receiving bore is defined by the core pin. Therefore, the front end of the ferrule tends to suffer from pronounced flow marks and shrinkage which distort the core wire receiving bore and impair the coaxiality of the core wire receiving bore with respect to the insert pipe. The outer circumferential surface of the insert pipe defines the outer diameter of the front end of the ferrule.
If the core pin is kept in the core wire receiving bore following the molding process until the thermal shrinkage has ceased so as to ensure a uniform inner diameter of the core receiving bore as a measure against the aforementioned problem. This will produce a uniform core wire receiving bore. However, the resulting thermal shrinkage will prevent the core pin to be removed, and forcing the removal of the core pin may damage and/or deform the core pin and core wire receiving bore. This will severely impair the production efficiency in mass production.
If communication holes are formed in the insert pipe for permitting the passage of injected plastic material, preparing the insert pipe creates a problem. Drilling the communication holes and maintaining the circularity of the insert pipe without deforming it is not easy. The grinding work for removing burrs that may form around each communication hole is also not easy. These factors contribute to the increase in the production cost of the insert pipe, and prevent the cost minimization in mass producing ferrules.
In view of such problems of the prior art, a primary object of the present invention is to provide a plastic ferrule and a method for molding the same that are free from such problems.
A second object of the present invention is to ensure a constant and uniform inner diameter to the core wire receiving bore provided in the front end of the ferrule.
A third object of the present invention is to ensure highly precise external dimensions and coaxiality by means of the process of injection molding using an insert pipe in a both easy and economical manner.
A fourth object of the present invention is to provide an optical fiber cable ferrule which can be mass produced in an efficient manner.
A fifth object of the present invention is to provide an improved method for making such an optical fiber cable ferrule.
According to the present invention, these objects can be accomplished by providing a plastic ferrule for an optical fiber cable, comprising: a substantially cylindrical main body made of molded plastic material; a core wire receiving bore formed axially centrally in a front end portion of the ferrule main body; a sheath receiving bore formed axially centrally in a rear end portion of the ferrule main body in axial alignment with and in communication with the core wire receiving bore, the core wire receiving bore having a smaller inner diameter than the sheath receiving bore; a flange portion extending radially and integrally from an axially intermediate part of the ferrule main body; and an insert pipe fitted on the front end portion of the ferrule main body; the insert pipe being provided with a coaxial small-diameter tubular portion in a front end thereof so as to form a thin-walled plastic portion between an inner circumferential surface of the small-diameter tubular portion and the core wire receiving bore.
Owing to the provision of the thin-walled plastic portion around the core wire receiving bore adjacent to the front end surface of the ferrule, the tapering of the core wire receiving bore due to thermal shrinkage following the injection molding process can be controlled. The thin-walled plastic portion has a relatively small mass as compared to the remaining part surrounding the core wire receiving bore, and therefore produces a relatively small shrinking force which mitigates the tendency of this part of the core wire receiving bore to diminish in diameter. Therefore, the inner diameter of the core wire receiving bore can be formed at a required precision so that an extremely fine optical fiber core wire can be inserted easily without causing cracks and deformations in the core wire, and various factors such as an increase in transmission loss and reflective attenuation and impairment of linearity that impair the performance of the connector can be avoided.
The small-diameter tubular portion can be formed in a number of possible ways, such as by drawing a front end portion of the insert pipe, machining an outer circumference of a front end portion of the insert pipe, and folding a front end of the insert pipe coaxially inward.
The small-diameter tubular portion can be formed also by forming the insert pipe with an outer pipe portion and an inner pipe portion received coaxially in the outer pipe portion, and projecting a front end portion of the inner pipe portion from a front end of the outer pipe portion. The outer and inner pipe portions may be closely fitted one in the other. According to another embodiment, a base end of the inner pipe portion is closely fitted in the outer pipe portion and a front end of the inner pipe portion is spaced radially inward from the outer pipe portion. According to yet another preferred embodiment of the present invention, the inner pipe portion extends substantially over an entire length of the ferrule main body, and defines an inner circumferential surface of the sheath receiving bore of the ferrule main body while the outer pipe portion defines an outer circumferential surface of the front end of the ferrule.
The inventors have also discovered that the front end of the flow of plastic material that is injected into the cavity contains a higher component of slag or other non-uniform part of the plastic material, and such a part undergoes a thermal contraction in a somewhat unpredictable manner. Therefore, if such a part is allowed to form the front end of the ferrule main body at which the core wire receiving bore opens out, the uniformity in the inner diameter of the core wire receiving bore would be impaired. It is therefore preferable to control the flow of the injected plastic material so as to prevent the front end of the flow of the plastic material from forming the front end of the ferrule main body.
Based on such a consideration, it is preferred that a front end of the small-diameter tubular potion terminates short of a front end surface of the ferrule main body, and the ferrule main body comprises a plastic reservoir formed around the front end of the small diameter tubular portion. The provision of such a plastic reservoir prevents the generation of pronounced flow marks and shrinkage which distort the core wire receiving bore adjacent to the front end and impair the coaxiality of the core wire receiving bore with respect to the insert pipe which defines the outer diameter of the ferrule. This in turn allows the ferrule to be connected to an opposing ferrule at a highly precise registration.
The plastic reservoir may be provided around the small-diameter tubular portion so as to communicate with the thin-walled plastic portion via the front end of the thin-walled plastic portion. This allows the prevention of the distortion in the inner diameter of the front end portion of the core wire receiving bore and the tapering of the core wire receiving bore both easily and economically.
If the insert pipe comprises an outer pipe portion and an inner pipe portion which are radially spaced from each other, the plastic reservoir may be formed in an axially middle part of the ferrule main body. For instance, the insert pipe may be designed in such a manner that the plastic material injected from the flange portion reaches the front end of the ferrule main body and, after turning around the front edge of the inner pipe portion, is directed toward the axially middle part of the ferrule main body. A slag component may safely form an axially middle part of the ferrule main body without creating any serious problem.
The method for molding a plastic ferrule according to the present invention may comprise the steps of: preparing a die assembly defining a substantially cylindrical cavity therein for molding the ferrule main body; placing a first core pin for defining the core wire receiving bore axially centrally in a first axial end of the cavity corresponding to the front end of the ferrule main body and a second core pin for defining the sheath receiving bore axially centrally in a second axial end of the cavity corresponding to the rear end of the ferrule main body, both in a retractable manner; placing an insert pipe coaxially adjacent to the first axial end of the cavity, the insert pipe including a small-diameter portion in an axial front end thereof; and injecting plastic material into the cavity from a gate provided in an axially intermediate part of the cavity.
According to a preferred embodiment of the present invention, the die assembly comprises a sprue and a passage communicating between the sprue and the cavity, the passage including an annular section coaxially surrounding a part of the cavity corresponding to the flange portion of the ferrule main body and a plurality of discrete passages extending radially between the annular section and the cavity part corresponding to the flange portion at a circumferentially regular interval. Alternatively, the radial passages may be replaced by a disk-shaped passage extending radially between the annular section and the cavity part corresponding to the flange portion. Thereby, the plastic material is conducted into the cavity substantially in a circumferentially uniform manner, and this also contributes to the precision in the dimensions and shape of the ferrule main body.
The insert pipe for the plastic ferrule of the present invention preferably extends along the outer circumference of the front end of the ferrule main body to a part of the flange portion or partly overlaps the flange portion. The base end of the insert pipe may extend into a part of a flange of the ferrule, and the gate may be provided adjacent to a part of the flange more rearward than the base end of the insert pipe.
The insert pipe may be formed with communication holes for passing plastic material similarly as the prior art. However, by eliminating the communication holes, the circularity of the outer circumferential surface of the insert pipe serving as the mating surface for a connector sleeve can be ensured without difficulty. Also, the grinding work is simplified, and the manufacturing cost of the insert pipe can be minimized so that the cost of the ferrule can be reduced by mass production.
The material for the insert pipe may be selected from any hard metallic materials such as stainless steel, titanium and fiber reinforced metals (FRM), ceramic materials such as zirconia, and heat resistance and dimensionally stable plastic materials such as polyimide resin, among other possibilities.